This invention relates to air duct apparatus for use in conjunction with air supply fan units, particularly such units designed for buildings or other large structures.
It is well known to provide an air supply system for a building, which system includes a main air supply duct, branch supply ducts and a fan unit. Often an air conditioning unit will form part of this system in order to cool the air that is being forced through the ducts. A problem often encountered with such systems is that the fan unit can produce a substantial noise and this noise can be carried through the ductwork and thereby be heard by persons in the building or structure. Not only is this a problem downstream of the fan unit, but it can also be a problem, at least in the immediate vicinity of the fan unit, on the upstream side since sound can travel out through the passageways that feed air to the fan unit. The noise created by the large fans in these systems is a particular problem in those buildings which must or should be kept reasonably quite, for example in hospitals and other buildings where the occupants are sleeping on a regular basis.
In addition to providing some noise attenuation, an air duct structure located downstream from a fan unit often is required to deliver the airflow from the fan to one or more air filters or perhaps to an air conditioning unit. In such cases it can be important for the air stream provided at the outlet of the duct structure to be relatively uniform across the width and height of the outlet. In this way, the amount of air flowing through each filter or each section of the filter, would be approximately the same.
In the construction of the duct structure located immediately downstream from a fan unit, it can be advantageous if the size of the air flow passageway is gradually increased from the inlet to the outlet of the duct structure. By increasing the size of the passageway in this manner, the air flowing through the passageway is allowed to expand gradually, thus permitting the velocity energy of the air to be recovered. As a result, the static pressure of the airflow is thereby increased. A gradual expansion of the size of the passageway is important in order to obtain maximum regain of air velocity pressure. By constructing the outlet duct structure in this manner, one could use a smaller size of fan motor to supply the same size of building than would otherwise be the case.
Another requirement of the duct structure located downstream from an air supply fan unit, is the frequent need to convert the airflow passageway from one having a round cross section at the outlet of the fan unit to one having a rectangular cross section. It will be appreciated that a rectangular air supply duct generally provides a more efficient use of the space available in a building for such ducts. Accordingly, it is often a requirement in a building that the air supply ducts and particularly the main ducts be substantially rectangular or square. The distance available to a duct designer or an air duct supplier for making this transition from a round cross-section to a rectangular one will vary from on job site to the next but, at least for some building sites, the transition distance can be quite short.
U.S. Pat. No. 4,418,788 issued Dec. 6, 1983 to Mitco Corporation describes a combined branch take-off and silencer unit for an air distribution system. This combined apparatus has two series-coupled sections, the first being a static pressure regain section and the second section having a main airflow passageway extending along its centre axis and branch ducts which connect smoothly with the main passageway. The structure is constructed with internal walls made of perforated metal sheets which overlays fibreglass packing provided for sound absorption. The main duct in this apparatus has a circular cross-section.
U.S. Pat. No. 4,295,416 issued Oct. 20, 1981 to Mitco Corporation describes a building air distribution system with a mixing plenum for receiving and mixing outside and return air. There is an input flow concentrator and integral silencer disposed within the plenum. The output port of this unit is connected to a fan unit which drives the air to the main duct of the building. The concentrator/silencer has inner and outer sections which are axially symmetrical about a vertical axis. It has an input port which extends symmetrically about this axis and a circular output port at the top. The inner and outer sections are lined with acoustically absorbing material.
U.S. Pat. No. 4,986,170 dated Jan. 22, 1991 issued to the present applicant describes a branch take-off airflow device which can be used immediately downstream of a fan unit. In the take-off section of the unit, the take-off passageways are rectangular in transverse cross-section whereas the main airflow passageway extending axially through the unit has a circular cross-section. In this main passageway there is an elongate airflow defining member which has a round, transverse cross-section with a maximum diameter equal to the diameter of the hub of the adjacent fan.
British patent No. 1,423,986 in the name of Alan Dodson et al, published Feb. 4, 1976, describes a silencer duct designed for use in a roof opening where an extractor fan is located above the opening. Opposite sidewalls of the duct are lined with sound absorbing material such as glass fibre slabs. Additional silencing is provided in the form of flow-splitter baffles which are flat and parallel to each other. This duct unit has a rectangular cross-section. The baffles themselves contain sound absorbing material.
It is an object of the present invention to provide improved air duct structure for both the inlet and the outlet sides of an air supply fan unit for a building or other large structure. Both the inlet and the outlet apparatus are provided with improved sound attenuating capabilities.
It is another object of the invention to provide a resonator mechanism for reducing the narrow band peak noise generated by the fan blade passages, which mechanism includes a hollow resonator chamber extending around or located adjacent to the inlet or the outlet that is connected to the fan unit.
It is a further object of the invention to provide an inlet duct apparatus or an outlet duct apparatus having at least first and second series of splitters with the splitters of each series being spaced apart to form smaller air passageways and mounted side-by-side in a row. The second series is positioned downstream in the airflow passageway relative to the first and is staggered with respect to the first series. In addition to improved sound attenuation, these splitters promote the regain of air velocity pressure in the unit.